1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus, and more particularly to a circuit for automatically correcting for a defective pixel of a solid-state image pickup apparatus for use as a video camera, for example, which is capable of switching between a field reading mode and a frame reading mode.
2. Description of the Prior Art
The quality of images produced by a video camera using a solid-state image pickup such as a CCD (Charge-Coupled Device) may be lowered by a defective pixel or pixels that produce a signal of a peculiar level even when no light falls thereon.
Heretofore, such a problem has been solved by a correcting circuit incorporated in a video camera for correcting a signal produced by a defective CCD pixel. More specifically, before manufactured video cameras are shipped to users, each camera is checked for any defective CCD pixels and information indicating the defective CCD pixels is stored in a memory area of the correcting circuit in the video camera. After such a video camera is delivered to a user, signals produced by the defective CCD pixels are corrected by the correcting circuit based on the stored information.
The signal correction is carried out by interpolation processes including zeroth-order interpolation and linear interpolation. According to the zeroth-order interpolation, a signal generated by a defective pixel is held by a sampling circuit, and replaced with a signal produced by a pixel which precedes the defective pixel. According to the linear interpolation, signals produced by respective pixels which precede and follow a defective pixel are averaged, and a signal generated by the defective pixel is replaced with the average signal.
The signal correction method according to the zeroth-order interpolation and linear interpolation will hereinafter referred as a first correction method.
Another signal correction method which does not rely on the interpolation processes controls the video camera to generate a signal corresponding to a signal that is produced by a defective pixel, and deduce the generated signal from the signal produced by the defective signal, thus canceling out the signal produced by the defective signal. This signal correction method will hereinafter referred as a second correction method.
CCD pixels may become defective during the manufacturing process and also may develop a sudden defect after the video camera is actually delivered to the user. Furthermore, CCD pixels may produce an aging-induced signal of a peculiar level when no light is applied thereto.
Such a sudden CCD pixel defect or an aging-induced CCD pixel defect cannot be corrected by checking the camera for any defective CCD pixels and storing information indicative of the defective CCD pixels in a memory area of the correcting circuit in the video camera before the video camera is delivered to a user.
One recent proposal includes a circuit in a video camera for detecting a defective CCD pixel. Any defective CCD pixel is detected by such a circuit, and information indicating the detected defective CCD pixel is stored in a memory in the video camera, so that a signal generated by the defective CCD pixel is corrected based on the stored information.
A defective CCD pixel is detected as follows: While no light is falling on the CCD, the level of a signal generated by each CCD pixel is compared with either the level of a signal generated by a preceding CCD pixel or a predetermined threshold level. If the level of a signal generated by a certain CCD pixel is larger than the level of the signal generated by the preceding CCD pixel or the predetermined threshold level, then that certain CCD pixel is detected as a defective CCD pixel which produces a signal of a peculiar level, and address data with respect to the defective CCD pixel and flaw (defective CCD pixel) data based on the signal level of the defective CCD pixel are produced and stored in the memory.
After the defective CCD pixel is detected, the signal of a peculiar level produced thereby, which is contained in a video signal generated by the CCD, is corrected according to either the first correction method or the second correction method.
It is known that there are two modes for reading a signal from a CCD, i.e., a field reading mode and a frame reading mode. Since many recent video cameras are capable of operating in both field and frame reading modes for reading CCD signals, it is necessary for such video cameras to detect any defective CCD pixels accurately. These two reading modes will briefly be described below with reference to FIGS. 1A, 1B and 2A, 2B of the accompanying drawings.
In the field reading mode, as shown in FIGS. 1A and 1B, electric charges are read from pixels in two vertically adjacent rows into respective-transfer regions (vertical transfer registers). The vertical transfer registers add electric charges from the pixels in two vertically adjacent rows, and then transfer the added electric charges to a horizontal transfer register.
More specifically, in an odd-numbered field, as shown in FIG. 1A, electric charges read from photosensitive regions I41, I31 are added by a transfer region, and electric charges read from photosensitive regions I21, I11 are added by a transfer region. Thereafter, the added electric charges are transferred to the horizontal transfer register. In an even-numbered field, as shown in FIG. 1B, electric charge read from the photosensitive region I41 and the region above (not shown) are transferred to and added by a transfer region, electric charges read from the photosensitive regions I31, I21 are transferred to and added by a transfer region, and electric charge read from the photosensitive region I11 and the region below (not shown) are transferred to and added by a transfer region. Thereafter, the transferred and added electric charges are transferred to the horizontal transfer register. In the field reading mode, since the electric charge of each pixel is read in every field, the motion resolution of the video camera is high. However, the vertical resolution is poor as the electric charges from the pixels in vertically adjacent rows are added to each other.
In the frame reading mode, as shown in FIGS. 2A and 2B, an electric charge is read from a pixel in one of two vertically adjacent rows in an odd-numbered field, and an electric charge is read from a pixel in the other row in an even-numbered field.
More specifically, in an odd-numbered field, as shown in FIG. 2A, electric charges read from photosensitive regions I31, I11, I32, I12 are transferred to transfer regions. In an even-numbered field, as shown in FIG. 2B, electric charges read from photosensitive regions I41, I21, I42, I22 are transferred to transfer regions. Thereafter, the transferred electric charges are transferred to a horizontal transfer register. In the frame reading mode, the vertical resolution is high because the electric charges from the pixels in vertically adjacent rows are not added to each other. However, since the electric charge of each pixel is read in every frame, the motion resolution of the video camera is low.
In view of the above shortcomings, the user of the video camera selects one of the field and frame reading modes with a selector switch depending on the conditions in which to use the video camera.